The three-dimensional positioning of a bonding tool, such as a bond head of a die bonder, directly affects the bonding quality. For a die bond head, its position affects the placement accuracy of a die during die bonding operations, as well as bond line thickness of the adhesive underneath the bonded die. Therefore, accurate control of the bonding tool's horizontal and vertical positions is vital. Unfortunately, due to external factors such as thermal expansion during the bonding operations, the three-dimensional position of the bond head during bonding would vary under different working conditions.
A few methods have been implemented in the prior art to address this problem. One approach is to install a temperature sensor near the bonding tool, but the measured temperature generally does not correlate well to the bonding tool's position when the machine restarts bonding operation after a period of inactivity.
Another type of positioning control technique is described in U.S. Pat. No. 5,909,837 entitled “Contactless Bonding Tool Heater”, which teaches the use of a radiant heating element opposite the bonding tool, the heating element being juxtaposed and separated from the tip of the bonding tool. The heating element serves to keep the bonding tool within a certain temperature range, and there is some attempt to calculate a compensation factor by estimating an amount of cooling of the bonding tool arising from the time and the length of the extension of the bonding tool away from its heating element. Nevertheless, this approach is not accurate because the temperature of the bonding tool itself is not monitored, and cannot be precisely controlled when the bonding tool is extended from the heating element.
Yet another approach is disclosed in U.S. Pat. No. 6,555,401 entitled “Method of Controlling Bond Process Quality by Measuring Wire Bond Features”. In this approach, a first image of a bond pad is obtained before bond attachment to find a center of the bond pad, and a bonding tool is instructed to bond material to the center of the bond pad, before a second image of the bonded material is obtained. The coordinates of the bonded material and the center of the bond pad are compared to calculate any offset that is present, and the offset can then be corrected. A shortcoming of this method is that it cannot measure the vertical position of the bonding tool, since only the horizontal offset of the material can be observed from the second image that is obtained.
Due to the inadequacies of the said prior art positioning control methods, the problem of accurately detecting the positional offset of the bonding tool in real time and correcting it during bonding operations is still unresolved.